Method and apparatus for capacitively regenerating tissue and bone

ABSTRACT

A system (10) is disclosed for facilitating the healing of traumatized tissue and broken or fractured bone. The system (10) establishes an electric field between a pair of electrodes (14) positioned on opposite sides of the patient site (12), resulting in the production of an alternating current having the desired frequency and amplitude characteristic in the tissue or bone. Specifically, the system (10) includes a resonator (32) formed by an inductor (36) coupled in series with the resistor (R1) and capacitor (C1) of an equivalent circuit (34) representing the patient site (12), the electrodes (14), and any gaps (30) therebetween. This resonator (32) also includes a capacitor (C2) designed to prevent spurious, high-frequency oscillations. The resonator (32) is operated by a free-running oscillator (16), which maintains the operation of the resonator (32) at its resonant frequency. The oscillator (16) includes a forward network (38), having a CMOS inverter (42), and a feedback network (40), including an open-loop operational amplifier (52), and provides a simple, stable, and efficient form of operation free from spurious, high-frequency oscillations.

FIELD OF THE INVENTION

This invention relates generally to the electric stimulation of tissueand bone and, more particularly, to the production of a stimulativeelectric current in the tissue or bone via a capacitively establishedelectric field.

BACKGROUND OF THE INVENTION

The use of electric current to facilitate the healing of traumatizedtissue and broken or fractured bone has been recognized for some time.The stimulative effect of such current appears to occur whether the flowof current is induced naturally, by internal body mechanisms, orartificially, by external sources. While the natural flow of currentproduced by electrochemical, myoelectric, and piezoelectric-like bodymechanisms advantageously facilitates healing without externalcircuitry, in some instances it is desirable to expedite therecuperative process by artificially supplementing the natural currentflow.

A variety of different techniques have been devised for establishingsupplemental electric currents in tissue and bone. Briefly, suchtechniques can be grouped according to both the type of currentdeveloped and the manner in which the current is established.Considering first the type of current produced, the current may becharacterized as either a direct (DC) current, having an amplitude thatis substantially constant as a function of time, or an alternating (AC)current, which exhibits a time-dependent amplitude variation. The use ofAC current is preferred because it can be established in a variety ofways, discussed below. DC current, on the other hand, can be inducedonly by providing a direct electrical connection between the tissue orbone and the energy source.

The manner in which the auxiliary stimulating current is induced offersseveral alternative forms of classification. First, such techniques canbe categorized as being either invasive or noninvasive, depending on theconnections provided to the patient. Invasive techniques involve theapplication of electric current directly to the site of the trauma orfracture through electrodes implanted at the site. While this approachminimizes the electric potential required to generate a particulardesired current in the tissue or bone, it also involves the expense andrisk of infection attendant surgical implantation procedures. As aresult, noninvasive procedures, in which the flow of electric current inthe tissue or bone is induced by apparatus external to the patient, arepreferred.

The noninvasive establishment of an AC current in tissue or bone can befurther grouped according to the electric principle involved in itsproduction. For example, a resistive approach involves the conduction ofcurrent directly to the patient through special electrodes coupled tothe patient's skin via conductive gel. This technique has thedisadvantage of requiring good electrical contact between the electrodesand the skin, necessitating the periodic replacement of the conductivegel on the electrodes.

A second, or inductive, approach employs magnetic fields to establishthe desired AC current in the tissue or bone. Specifically, thisapproach involves the application of an electric current to magneticcoils positioned proximate the patient. The flow of current through thecoils produces a magnetic field in the patient's bone or tissue,resulting in the establishment of the desired alternating therapeuticcurrent. This approach has a number of disadvantages including therequired use of bulky magnetic coils and an energy source having anoutput whose frequency and waveform are sufficient to induce the desiredstimulating currents at the patient site. The inductive approach alsoinvolves relatively large energy losses attributable to the heating ofthe coil windings produced by the flow of current therethrough.

The third technique for noninvasively establishing an alternatingcurrent in the patient's tissue or bone can be referred to as the"capacitive" or electric field approach. This technique typicallyemploys a pair of electrodes that are placed on opposite sides of thetreatment site and are insulated from the patient's skin. Energy appliedto the electrodes establishes an electric field between the electrodes,normal to the skin. It is this electric field that induces thealternating therapeutic current at the treatment site.

While this approach overcomes the difficulties outlined above withrespect to the resistive and inductive techniques, the capacitiveproduction of therapeutically effective levels of electric current atthe patient treatment site traditionally presents several additionalproblems. For example, the patient's skin normally contributes a seriescapacitive reactance to the equivalent electric circuit representativeof the elements between the electrodes. In addition, a much largervariable capacitive reactance is exhibited by the insulative, dielectric"gaps" between the electrodes and the patient.

The combined series capacitive reactance of the elements between theelectrodes has been a problem for several reasons. First, this reactivecomponent seriously attenuates the current flow produced by a givenpotential applied to the electrodes. To understand how this energy lossarises, it may be helpful to briefly review the dynamics of interactionbetween the stimulating current and the patient. To be therapeuticallyeffective, the electric current must have a frequency that is low enoughto ensure its penetration to the site of the fracture or trauma. Thisrequirement is imposed because a high-frequency current will concentratenear the dermal region of the patient in response to a mechanism knownas the "electromagnetic skin effect." Other, biological, mechanisms thatlimit the efficacy of relatively high-frequency current also likelyexist.

At therapeutically effective frequencies, the peak energy storedelectrostatically in both the electrode-to-skin interface and theepidermis during one cycle of the alternating voltage potential appliedto the electrodes is substantially larger than the energy absorbed bythe tissue and bone being treated. This stored energy is typicallyeither dissipated within the source or radiated as electromagneticenergy, resulting in a system inefficiency or energy loss. As a result,high levels of reactive power are required to establish the desiredtherapeutic current.

As an alternative to the use of higher voltages to compensate for energylosses, an inductive reactance can be employed to produce a resonantcircuit that reduces the energy losses. For example, a series-connectedinductor can be used to recapture the stored energy and apply it to thetreatment site during the next cycle of the alternating voltage appliedto the electrodes. As a result, only a relatively small amount of energyis dissipated and radiated.

Even with energy losses reduced, the capacitive technique ofestablishing therapeutic current in tissue and bone still presentsseveral problems. As noted previously, the capacitive reactanceexhibited by the dielectric gaps between the electrodes and the patientrepresents a rather large and variable electrical impedance to the drivecircuit. The addition of the inductor to form a resonant circuit havinga high quality factor Q, provides a significant reduction in impedancewhen the circuit is operated at its resonant frequency. Because thecapacitive portion of the circuit may vary substantially in response toboth the condition of the patient and movement between the electrodesand patient, when a fixed inductance is employed the circuit can bemaintained at resonance only by adjusting the frequency of the driver tocorrespond to the resonant frequency of the circuit as it varies withthe changing capacitance. Alternatively, a variable inductive reactancecan be used to negate the effect of the changes in capacitance, leavingthe resonant frequency of the circuit unchanged.

In U.S. Pat. No. 4,459,988 (Dugot) a circuit is disclosed that employsthe former technique. Specifically, a portion of the patient positionedbetween stimulating electrodes is included in a series resonant circuitincorporating positive feedback to maintain the frequency of thestimulating signal at the resonant frequency of the circuit. The Dugotapproach, however, suffers from several disadvantages. First, thedisclosed implementation is relatively complex and involves a largenumber of components. Positive feedback is required to stabilize thecircuit with respect to frequency and automatic gain control ispreferably employed to regulate the amplitude of the signal generator'soutput. In addition, the output produced by the circuit may be subjectto spurious and multiple high-frequency oscillations decreasing theefficiency of the system. The use of a variable inductive reactance tomaintain a constant resonant frequency in the presence of capacitivechanges disadvantageously requires the use and expense of some form ofadjustable inductor and feedback to control it.

In light of the preceding remarks, it would be desirable to provide amethod and apparatus for noninvasively establishing a regenerativeelectric current in traumatized tissue and broken or fractured bone. Itwould further be desirable to employ a capacitive technique ofestablishing such a current that is simple, does not require ahigh-voltage potential to overcome large variable gap capacitances, isinherently stable with respect to both frequency and amplitude, andrejects high-frequency spurious oscillations.

SUMMARY OF THE INVENTION

In accordance with this invention, a method and apparatus are providedfor capacitively establishing an alternating electric field between apair of stimulation electrodes. The electrodes are typically separatedby a patient region that includes traumatized tissue and/or broken orfractured bone and by dielectric gaps between the electrodes and thepatient. The electric field is designed to produce an alternatingcurrent in the tissue or bone to accelerate healing and is establishedby a free-running oscillator. Oscillation is maintained with the aid ofa resonator that includes an inductor and the resistive and capacitivecomponents provided by the patient, electrodes, and gaps. The oscillatorhas a simple construction, designed to operate in an amplitude andfrequency-stable manner, with its frequency of oscillation trackingchanges in the resonant frequency of the resonator attributable to, forexample, variations in the capacitance of the dielectric gaps. Theoscillator is further constructed to limit the occurrence of spurioushigh-frequency oscillations.

In accordance with a particular aspect of this invention, an apparatusis provided for applying an electric current to a region of a patientthrough a pair of electrodes positionable in noncontacting relationshipwith respect to the patient. This region of the patient, and any gapsbetween the patient and the electrodes, exhibits a series capacitanceand resistance. The apparatus includes an inductor, coupled to one ofthe electrodes to define a resonator in cooperation with the seriescapacitance and resistance. An oscillator is coupled to the inductor toprovide to the resonator a periodic current having a frequency that issubstantially equal to the resonant frequency of the resonator. Finally,an element is included to limit the occurrence of spurious oscillationfrequency in the periodic current provided by the oscillator.

In accordance with another aspect of this invention, an apparatus isprovided for establishing an electric field between a pair of electrodesseparable by a gap that exhibits a series capacitance and resistance.The apparatus includes an inductor connectable in series with one of theelectrodes to define a resonant circuit with the series capacitance andresistance. A low-impedance voltage source is included to apply a squarewave output, shifted in phase by approximately 180 degrees from thesource input, to one of the electrodes. The other one of the electrodesis coupled to the input of an open-loop operational amplifier whoseoutput is coupled to the voltage source. The operational amplifier has again sufficient to provide a unity gain for the apparatus and produces aphase shift sufficient to provide a zero-degree phase shift for theapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will presently be described in greater detail by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a system, constructed in accordance withthis invention, that capacitively regenerates the tissue and bone of apatient;

FIG. 2 is a schematic diagram of a an oscillator circuit, employed bythe system of FIG. 1, which provides a periodic current to the patientthrough a pair of electrodes separated from the patient by gaps;

FIG. 3 is a more detailed schematic diagram of the embodiment of theoscillator circuit shown in FIG. 2; and

FIG. 4 is a schematic diagram of a second embodiment of the oscillatorcircuit of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a system 10 for facilitating the healing oftraumatized tissue and broken or fractured bone is illustrated. Thesystem 10 establishes an alternating electric current at a treatmentsite 12, which includes the bone and tissue for which enhancedregeneration is desired. As discussed in greater detail below, thistherapeutic current is induced by the establishment of an electric fieldbetween a pair of electrodes 14 positioned on opposite sides of the site12. A free-running oscillator 16, included in a control and outputsubsystem 18, provides the energy required to maintain the field betweenelectrodes 14. Oscillator 16 is powered by a supply 20 and iscontrollably activated and deactivated in response to a timer 22. Avisual display 24, included in subsystem 18, provides an outputindicating that a therapeutic current is flowing at the treatment site12. As additional outputs, subsystem 18 includes a timing pulse counter26 to provide information concerning the length of time during whichtherapeutic current is applied to the site 12 and an alarm 28 thatindicates that supply 20 is no longer able to sustain the desiredoperation of oscillator 16.

While the operation of system 10 is described in greater detail below,its primary function is to establish a therapeutic current at site 12.In the preferred arrangement, an alternating therapeutic current ofapproximately 5 milliamperes is produced. The frequency of thealternating current is a function of a variety of factors, including thestructure of site 12, electrodes 14, and free-running oscillator 16, aswell as the relative position of site 12 and electrodes 14.

While a therapeutic effect is produced by induced currents having arelatively wide range of frequencies, the current's frequency should besufficiently high to prevent the deleterious electromigration ofisotopes or ionic species at the treatment site 12. On the other hand,because a "conductor" such as site 12 exhibits a "skin effect" at highfrequencies that causes current to concentrate near the surface of theconductor, the frequency must be sufficiently low to ensure that currentis induced, for example, in the fracture of a bone lying well beneaththe surface of the patient's skin. It has been found that theestablishment of an alternating current with a nominal frequency ofapproximately 50 kilohertz provides the desired therapeutic effect forall normally expected patient and electrode conditions and ensures thatthe frequency will remain within an acceptable range as the oscillator16 responds to varying patient and electrode conditions.

Turning now to a more detailed discussion of the various components ofsystem 10, the electrodes 14 are constructed to satisfy a variety ofoperating criteria. For example, because the current-inducing electricfield is established normal to the surface of electrodes 14, the area ofelectrodes 14 directly affects the cross-sectional area of the treatmentsite 12 in which a therapeutic current is produced. In addition, theconstruction details of electrodes 14, including their size and thematerials employed, will influence the capacitive reactance introducedby the electrodes. In the preferred arrangement, electrodes 14 arecircular plates of conductive elastomer, such as carbon-filled siliconerubber, having a diameter of 8 cm, a thickness of 0.1 cm, and a typicalcapacitance of 50 pF.

As indicated in the block diagram of FIG. 1, the electrodes 14 arepreferably spaced apart from the treatment site 12 by gaps 30. The gaps30 can be physically maintained by the inclusion of a rigid dielectricmaterial between the electrodes 14 and site 12. For example, in apreferred arrangement, the electrodes 14 are embedded in a cast thatsurrounds the traumatized tissue and broken or fractured bones. The gaps30 then include both the dielectric cast material and any air gapinterposed between electrodes 14 and the treatment site 12 of thepatient. Because the length of the air gaps will likely varysubstantially in response to, for example, patient movement, theequivalent capacitive reactance of the circuit may undergo significantvariations. It is these variations that the free-running oscillator 16is designed to compensate for, resulting in the production of anacceptable therapeutic current at the site 12 under all normallyexpected conditions.

Turning now to a more detailed discussion of the free-running oscillator16, reference is had to FIG. 2. As shown, the oscillator 16 operates inconnection with a series resonant circuit or resonator 32 defined by anequivalent site circuit 34 and an inductor L1. The equivalent circuit 34includes a resistive component R1 and a capacitive component C1 that arerepresentative of site 12, electrodes 14, and gaps 30. As discussed ingreater detail below, the oscillator 16 is constructed to ensure thatits operating frequency precisely tracks the changing resonant frequencyof resonator 32. Oscillator 16 is further constructed from a minimum ofcomponents, providing a stable level of injected current over a desiredfrequency range without the introduction of high-frequency spuriousoscillations.

As noted, free-running oscillator 16 is designed to operate resonator 32at its resonant frequency. Resonance is, by definition, the condition inwhich the impedance of the resonator 32 is purely resistive, causing thevoltage and current at the input of resonator 32 to be in phase. Becausethe inductive and capacitive components L1 and C1 do not affect thecurrent flowing through resonator 32 in this condition, the magnitude ofthe current is a function only of the voltage applied to resonator 32and the resistance of resonator 32.

To ensure that the free-running oscillator 16 operates at the resonantfrequency of resonator 32, two conditions characteritic of the stableoperation of any oscillator must be satisfied. First, the closed-loopgain of the circuit defined by the oscillator 16 and resonator 32 mustbe equal to unity. Addressing this condition in greater detail, thecircuit shown in FIG. 2 can be considered to include a forward network38 and a feedback network 40. The forward network 38 includes anamplifier 42 that amplifies the output V₁ of feedback network 40 by acomplex, frequency-dependent gain A to produce an output voltage V₀. Thefeedback network 40 includes the resonator 32 and feedback elements 46.The feedback network 40 is characterized by a complex,frequency-dependent transfer function β and produces the output V₁,which is equal to the product of voltage V₀ and the transfer function β.

The loop gain of the circuit in FIG. 2 is equal to the product of theterms A and β. As this loop gain approaches +1, the ratio of thecircuit's output voltage to its input voltage approaches infinity,allowing the circuit to oscillate or produce an output even when noinput is applied. Both A and β are complex, frequency-dependentquantities that can be expressed in polar form as an amplitude and anangle. Because oscillation can take place only if the vector product ofA and β is +1, the product of the amplitudes of A and β must be equal to+1, while the sum of the angles of A and β must equal 0. This lattercharacteristic defines the second requirement for oscillation, which isthat the loop phase shift must be equal to 0 or some multiple of 360degrees.

Discussing the various components of the basic circuit of FIG. 2 ingreater detail, resonator 32 can be considered to include four elements.As noted previously, the equivalent circuit 34, representative of site12, electrodes 14, and gaps 30, includes a series capacitance C1 andresistance R1. The portion of C1 attributable to the gaps 30 isrelatively large and variable, leading to fluctuations in the resonantfrequency of resonator 32. The equivalent series resistance R1 includesthe resistance inherent in the living tissue, which is on the order of100 ohms. This resistance R1 also includes the effective seriesresistance associated with the forward network 38 and feedback network40, as well as the losses in the series inductor L1. This lattercomponent may be made relatively small by carefully designing theinductor L1 to provide a high quality factor Q.

As shown in FIG. 2, resonator 32 also includes a capacitor C2 connectedin series with equivalent circuit 32. Capacitor C2 prevents oscillator16 from operating in spurious and multiple high-frequency modes byintroducing a high-frequency roll-off into the feedback network 42. Thecapacitance (e.g., 10 nF) of capacitor C2 is typically much greater thanthat of C1. One end of capacitor C2 is coupled to ground, while theother end is connected directly to equivalent circuit 34 and by feedbackto the forward network 38. The series inductor L1 is included inresonator 32 to reduce the dissipation of energy from the electric fieldestablished between electrodes 14 by receiving most of the stored energyof the field and returning it during the next cycle of the alternatingpotential applied to the electrodes 14. Inductor L1 typically has aninductance on the order of 100 mH.

Turning now to a discussion of the forward network 38, network 38,preferably includes a source 42 that provides a low-impedance outputvoltage to resonator 32. The voltage source 42 also preferably operatesin a switching mode for enhanced efficiency. A suitable source oramplifier 42 is a complementary, metal-oxide-semiconductor inverter ofthe type manufactured by Motorola under the designations MC14049UB orMC14069UB. Such an amplifier 42 provides a square wave output andintroduces a phase shift of approximately 180 degrees into the loop atthe desired operating frequency of around 50 kilohertz.

Forward network 38 also includes a resistor R2. As noted previously, atresonance, the capacitive and inductive reactances have a cancellativeeffect on each other, exerting no influence on the amplitude of thecurrent flowing through resonator 32. As a result, the therapeuticcurrent induced at the patient site 12 can be controlled either byaltering the voltage applied to resonant circuit 32 or its seriesresistance. The resistor R2 is connected in series with the equivalentresistance R1 of resonator 32 to regulate the therapeutic current to thedesired level. Preferably, R2 has a resistance (e.g., 750 ohms) that issufficient to increase the total series resistance of the circuit toapproximately 1000 ohms. As will be discussed in greater detail below,with a compact, 9-volt battery for source 20 and the 4049-type inverter42 employed in forward network 40, an oscillator output of approximately5 volts rms is achieved, providing a normally desired therapeuticcurrent of 5 milliamperes at resonance.

As shown in FIG. 2, feedback network 40 includes both the resonator 32and feedback elements 46. Feedback elements 46 provide the loop with thedesired unity gain and zero phase shift characteristics. While elements46 could be merged with the forward network 38, both in theory and inpractice, they are shown separately in FIG. 2 to assist in anunderstanding of the circuit.

As represented in FIG. 2, the feedback elements 46 include an amplifier48 and phase lag 50, which ensure that the complex transfer function βof the feedback network 40 satisfies the loop's unity gain and zerophase shift requirements. Specifically, the gain provided by amplifier48 compensates for both the gain of amplifier 42 and the attenuationintroduced by resonator 32. Regarding phase shift, the current flowingthrough the resonator 32 is in phase with the voltage at the output ofthe forward network 38 at resonance. This current produces a voltageacross capacitor C2 having a phase that lags that of the output voltagefrom amplifier 42 by 90 degrees. Assuming that amplifiers 42 and 48introduce a phase shift of 270 degrees into the loop, phase lag 50 isrequired to provide the remaining 90 degrees of shift. As a result, 360degrees of phase shift is produced, allowing stable oscillation.

To ensure that oscillator 16 will oscillate at start-up, the combinedgain of amplifiers 42 and 48 is designed to exceed any voltageattenuation introduced by the resonator 32 and phase shifter 50. Theunity gain condition for stable oscillation of the loop is satisfied bymaking at least one of the amplifiers 42 or 48 saturable. With a4049-type device employed for amplifier 42, such saturation is inherent.

FIG. 3 illustrates, in slightly greater detail, a preferred embodimentof the system 16 of FIG. 2. Addressing first the resonator 32, thecapacitor C1 of FIG. 2 has been depicted as two capacitors C1_(a) andC1_(b) in FIG. 3. The capacitance of element C1_(a) corresponds to thatof the electrodes 14 and gaps 30, while capacitor C1_(b) isrepresentative of site 12.

The series resonant circuit 32 of FIG. 3 also includes an additionalcapacitor C3, having a capacitance on the order of 220 picofarads. Whilethe inclusion of this capacitor C3 is not essential, it assuresoscillation of system 10, regardless of the normal variations ofcapacitance of elements C1_(a) and C1_(b). As will be appreciated, theseries capacitance of elements C1_(a) and C1_(b) may become very largeand even representative of a virtual short circuit when, for example,the patient is perspiring heavily and the electrodes 14 are spaced apartfrom the patient by only a woven cotton sleeve designed to "wick"perspiration from the site. As a result of this increase in capacitance,the resonant frequency of resonator 32 could shift sufficiently toprevent oscillator 16 from achieving a unity loop gain or zero phaseshift and, hence, oscillation. By adding a capacitor C3 whosecapacitance is preferably on the order of four times that of the nominalvalue of the series combination of capacitors C1_(a) and C1_(b), theresonant frequency of resonator 32 cannot drop to less thanapproximately one-half its nominal value and oscillation is assured. Itis also preferable to use the additional capacitor C3 to prevent thepossible flow of direct current through the electrode circuit. Such flowmight have adverse consequences through electrolytic action, especiallyin the presence of moisture on the electrodes.

FIG. 3 also includes additional details regarding the construction ofthe feedback elements 46. As noted previously, this portion of network42 is designed to satisfy the zero phase shift and unity gainrequirements of oscillation by introducing a 90-degree phase lag intothe circuit, as well as sufficient gain to overcome the loss introducedby the resonator 32. Both functions are conveniently provided by anoperational amplifier 52 connected "open-loop" or without feedback. Theoperational amplifier 52 should have a sufficient open-loop gain overthe range of expected operating frequencies to make the oscillator loopgain equal to unity without depending upon amplifier 42 for gain. Asuitable operational amplifier 52 is provided by any one of the fouroperational amplifiers included in the integrated circuit devicemanufactured by Texas Instruments under the part designation TLC27M4.This device is available with internal compensation to provide a90-degree phase shift over a wide frequency range, including the desiredoperating frequency of approximately 50 kilohertz.

The inverting input of the operational amplifier 52 is coupled to avoltage divider formed by the series combination of resistors R3 and R4.More particularly, one end of resistor R3 is coupled to the supply 20,one end of resistor R4 is coupled to ground, and the connection ofresistors R3 and R4 is coupled to the inverting input. Because bothresistors R3 and R4 have a resistance that is on the order of 100kilohms, a voltage equal to approximately one-half the supply 20 voltageV_(dd) is applied to the inverting input. The inverting input ofoperational amplifier 52 is also coupled to ground by a bypass capacitorC₄ having a capacitance of 100 nF and the noninverting input is coupledto the resonator 32 at the ungrounded side of capacitor C2.

DC negative feedback is introduced into the oscillator circuit by aresistor R5. Resistor R5 is coupled between the noninverting input ofoperational amplifier 52 and the output of the amplifier 42 in forwardnetwork 38, as shown in FIG. 3. Resistor R5 has a resistance ofapproximately 100 kilohms and is included to ensure the initiation ofoscillation by placing amplifiers 42 and 52 within their common-moderanges at start-up.

Turning finally to a more detailed discussion of the forward network 38,as discussed previously it preferably employs a complementarymetal-oxide-semiconductor inverter for amplifier 42. The amplifier isdriven rail-to-rail at the voltage V_(dd) provided by supply 20 toenhance efficiency and is also coupled to a bypass capacitor C5 having acapacitance of approximately 100 nF.

As noted previously, resistor R2 is included in the forward network 38and has a resistance that is sufficient to provide the desiredtherapeutic current level for the particular output produced byamplifier 42. A pair of reverse, parallel-connected light-emittingdiodes D1 and D2, such as those manufactured by Texas Instruments underthe part number TIL213-2 are included in series with resistor R2. Thediodes D1 and D2 output an easily observable quantity of light whenenergized at the therapeutic current level of approximately 5milliamperes rms. As a result, the light-emitting diodes D1 and D2directly indicate the application of therapeutically effective currentto the site 12, rather than simply indicating that circuit power isavailable to establish such a current. This advantageously avoids theproduction of an output if oscillation ceases due to the failure of acomponent or an improper spacing of the electrodes 14.

Because each light-emitting diode D1 and D2 only passes current in onedirection, the reverse, parallel connection of diodes D1 and D2 isemployed to accommodate the alternating current established in theresonant circuit 32. The forward drop of diodes D1 and D2 is on theorder of one volt. As a result, the effective voltage applied toresonator 32 is reduced, requiring that the resistive value of resistorR1 be adjusted accordingly to maintain the desired level of therapeuticcurrent.

As will be appreciated, a number of modifications can be made to theoscillator circuit 16 of FIG. 3. For example, as shown in FIG. 4, theactive operational amplifier 52 can be removed from the circuit and aresistor R6 and capacitor C6 employed as the feedback elements 46. Withcapacitor C2 exhibiting a high reactance, for example, at a capacitanceof 1 nanofarad, the voltage across capacitor C2 will remain relativelyhigh. Thus, even after being attenuated by the combination of resistorR6 and capacitor C6, the voltage will still be sufficient to driveamplifier 42. The combination of resistor R6 and capacitor C6 alsoproduces the desired phase shift of approximately 90 degrees for allfrequencies that are attenuated by roughly a tenfold factor or more.

Another modification relates to the use of amplifier 42. While a singleamplifier 42 is shown in the schematic diagram of FIG. 3, a number ofsuch elements are normally connected in parallel to achieve an internalseries resistance that is relatively low in comparison to that of R2. Aswill be appreciated, the amplifier resistance is heavily dependent uponambient temperature and varies from unit to unit in a production run.Because the series resistance of the circuit directly affects currentlevel, the resistance of resistor R2 should always be dominant if thetherapeutic current level is to be accurately maintained.

Turning now to a discussion of the remaining elements of control andoutput subsystem 18, as noted previously, a 9-volt battery mayconveniently be employed for power supply 20. The use of such a batterynot only ensures the absence of any high-voltage failure modes, it alsocontributes to the relative portability of system 10, which may beparticularly desirable when system 10 is mounted to a patient's cast forextended periods of use.

Because the level of therapeutic current induced at site 12 is afunction, in part, of the voltage applied to resonator 32, it isimportant that supply 20 maintains a sufficient voltage for availabilityto oscillator 16. In this regard, a power supply voltage output 28 isincluded to provide an output indicative of the status of supply 20. Forexample, a simple direct current voltmeter could be employed for output28 to indicate the voltage available from supply 20. Alternatively,output 28 could include a comparator having the battery voltage as oneinput, a threshold voltage as another input, and an output coupled to anaudible or visual alarm when the battery voltage drops below thethreshold.

Subsystem 18 also includes a timer 22 designed to sequence oscillator 16on and off at desired intervals. More particularly, with empiricalstudies conducted to determine the cycling rate resulting in theproduction of an optimal therapeutic effect, timer 22 can then be set tocycle oscillator 16 on and off at that rate. In addition, timer 22 canbe set to initiate and interrupt this cycled operation at desired times.

As will be appreciated, various constructions can be employed for timer22, depending on the particular operation desired. For example, if anadjustable start time, stop time, and cycle rate are desired, along withthe ability to retain output information regarding the treatment period,a microprocessor-based timer 22 programmed with an appropriate set ofoperating instructions may be useful. In this manner, a timing pulseoutput 26 can easily be provided, displaying information indicative ofthe number of timing pulses applied to oscillator 16, for analysis bythe physician. Alternatively, output 26 can be a simple counter.

As noted previously, the visual display 24 preferably includes a pair ofreverse, parallel-connected, light-emitting diodes D1 and D2, whichdirectly indicate the establishment of a therapeutic current at the site12. As will be appreciated, with timer 22 set to cycle oscillator on forone second and off for two seconds, as an example, diodes D1 and D2 willappear to be lit and extinguished for corresponding intervals. In apreferred arrangement employing an appropriately programmed,microprocessor-based timer 22, when a low voltage is sensed at supply20, the cycle rate produced by timer 22 can be altered to provide achange in the display produced by diodes D1 and D2 indicating alow-battery condition.

The system 10 constructed in the manner described above has a number ofadvantages. For example, the system 10 induces the desired therapeuticcurrent at the patient site 12 in a straightforward manner withrelatively few components. The system 10 is also stable, rejectshigh-frequency spurious oscillations, and produces an output directlyindicating the establishment of a therapeutic current at the treatmentsite 12.

Another advantage of system 10 is that it enhances patient safety byavoiding the application of a high-voltage drive output directly to theelectrodes 14. More particularly, with a constant drive voltageemployed, the current established at the treatment site 12 varies ininverse proportion to the capacitive reactance of the equivalent circuit34. In certain circumstances, for example, when electrodes 14 come intodirect contact with the skin of a heavily perspiring patient, thecapacitive reactance may be negligible, resulting in the production of alarge and potentially injurious "fault" current.

The disclosed system 10 overcomes this difficulty by providing a drivevoltage that is limited, in case of direct electrode-to-skin contact, tothe relatively low voltage V_(dd) of the supply 20, for example, 9volts. As a result, system 10 has no high-voltage failure modes.Further, the drive current is limited by resistor R2, which may be aseries of several resistors or a single resistor constructed to haveonly an open-circuit failure mode. In no circumstance will any failureof an active component result in an over-current condition hazardous tothe patient.

Those skilled in the art will recognize that the embodiments of theinvention disclosed herein are exemplary in nature and that variouschanges can be made therein without departing from the scope and spiritof the invention. In this regard, and as was previously mentioned, theinvention is readily embodied with either active or passive componentsin the feedback network to provide the desired unity loop gain in zerophase shift. Further, it will be recognized that a variety of activeelements can be employed in the forward and feedback networks. Becauseof the above and numerous other variations and modifications that willoccur to those skilled in the art, the following claims should not belimited to the embodiments illustrated and disclosed herein.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An apparatus forapplying an electric current to a region of a patient through a pair ofelectrodes positionable in noncontacting relationship with respect tothe patient, the region of the patient and any gaps between the patientand electrodes exhibiting a series capacitance and resistance, saidapparatus comprising:inductive means, coupleable to one of theelectrodes, for defining a resonator in cooperation with the seriescapacitance and resistance; oscillation means, coupled to said inductivemeans, for providing to said resonator a periodic current whosefrequency is substantially equal to the resonant frequency of saidresonator, said oscillation means defines a closed loop with saidresonator and further comprises a first section means, for providing afirst amplitude adjustment and first phase shift to signals conducted bysaid loop, said first section means comprises a complementarymetal-oxide-semiconductor inverter, said first phase shift beingapproximately equal to 180 degrees, and a second section means forproviding a second amplitude adjustment and second phase shift tosignals conducted by said loop, said closed loop having a gain of unityand a phase shift equivalent to zero degrees; and means for limiting theoccurance of spurious oscillation frequencies in said periodic currentprovided by said oscillation means, wherein said means for limiting theoccurance of spurious oscillation frequencies is a capacitive elementfurther defining said resonator in cooperation with said inductive meansand said series capacitance and resistance, said capacitive elementintroducing an approximately 90-degree phase shift into said loop. 2.The apparatus of claim 1, wherein said second section means comprises anoperational amplifier, said second phase shift being approximately equalto 90 degrees.
 3. The apparatus of claim 2, further comprising resistivecurrent control means, coupleable to said resonator, for controlling theamplitude of said periodic current provided to said resonator.
 4. Theapparatus of claim 3, further comprising visual output means forproducing a visual output indicative of the provision of said periodiccurrent to said resonator.
 5. The apparatus of claim 4, wherein saidvisual output means comprises a pair of reversed, parallel-connected,light-emitting diodes coupleable in series with said resonator.
 6. Anapparatus for applying an electric current to a region of a patientthrough a pair of electrodes positionable in noncontacting relationshipwith respect to the patient, the region of the patient and any gapsbetween the patient and electrodes exhibiting a series capacitance andresistance, said apparatus comprising:inductive means, coupleable to oneof the electrodes, for defining a resonator in cooperation with theseries capacitance and resistance; oscillation means, coupled to saidinductive means, for providing to said resonator a periodic currentwhose frequency is substantially equal to the resonant frequency of saidresonator; visual output means for producing a visual output indicativeof the provision of said periodic current to said resonator, said visualoutput means comprising a pair of reversed, parallel-connected,light-emitting diodes coupled in series with said resonator; and meansfor limiting the occurance of spurious oscillation frequencies in saidperiodic current provided by said oscillation means.
 7. An apparatus forapplying an electric current to a region of a patient through a pair ofelectrodes positionable in noncontacting relationship with respect tothe patient, the region of the patient and any gaps between the patientand electrodes exhibiting a series capacitance and resistance, saidapparatus comprising:inductive means, coupleable to one of theelectrodes, for defining a resonator in cooperation with the seriescapacitance and resistance; oscillation means, coupled to said inductivemeans, for providing to said resonator a periodic current whosefrequency is substantially equal to the resonant frequency of saidresonator; means for limiting the occurance of spurious oscillationfrequencies in said periodic current provided by said oscillation means;a voltage source for operably powering said oscillation means; and anoutput means for providing a first output indicative of the provision ofsaid periodic current to said resonator when the voltage available fromsaid source is above a predetermined level and a second output when thevoltage available from the source is below the predetermined level. 8.An apparatus for establishing an electric field between a pair ofelectrodes separable by a gap that exhibits a series capacitance andresistance, said apparatus comprising:inductive means, connectable inseries with one of the electrodes, for defining a resonant circuit withthe series capacitance and resistance; low-impedance voltage sourcemeans for applying a square wave output, shifted in phase byapproximately 180 degrees from a source input, to the resonant circuit;and feedback circuit means, having an input coupleable to the resonantcircuit and an output coupled to said voltage source, said feedbackcircuit means being for providing a unity gain for said apparatus andproducing a phase shift sufficient to provide a zero-degree phase shiftfor said apparatus.
 9. The apparatus of claim 8, wherein the electricfield established exhibits a stable frequency and amplitude.
 10. Theapparatus of claim 8, further comprising capacitive means coupleable tosaid inductive means as part of said resonant circuit for limiting theoccurrence of spurious oscillation frequencies in the electric field.